Printing blankets are generally formed of several layers including an upper ink transfer or printing layer, a compressible or deformable middle layer and a lower carcass layer.
The carcass layer is generally formed of several layers of woven fabric bonded together by adhesive.
The carcass fabric is typically formed of natural, synthetic or mixed fibers. The fabrics are normally highly stretched in the machine (warp) direction. These fabrics are desirable in that they tend to produce blankets having low levels of elongation or stretch around the blanket cylinder during its use on a printing press. The use of fabrics with low machine direction elongation reduces the need for periodically tightening the blankets on a cylinder.
Unfortunately, these low machine direction elongation fabrics have a very high cross machine (fill) direction elongation characteristic. This is due, in large part, to the design of the fabric, namely that the machine direction fibers lie in a coplanar relationship to each other and the cross machine direction fibers follow a sinusoidal pattern over and under the machine direction fibers. This sinusoidal pattern results in a fabric having a high level of cross machine direction elongation at even low levels of force.
Cross machine direction elongation is a problem in that it causes the blanket to stretch and expand along the edges which reduces the print quality along the blanket edges. Typically, this problem has been eliminated by reducing the print width or using an oversized blanket and cylinder to achieve the desired print width. Either alternative is costly in that it underutilizes the paper and/or machine capacity.
Another alternative is to use a fabric having a higher machine direction elongation characteristic and therefore a corresponding lower cross machine direction elongation characteristic. This, however, is not acceptable as an increase in the machine direction stretch of the blanket requires more frequent tightening of the blanket and therefore a greater amount of downtime.
A further alternative is to add to the blanket one or more layers of monofilaments rods in a cross machine direction, such as shown in U.S. Pat. No. 4,224,370. This however substantially increases the overall thickness of the blanket and decreases the resiliency of the blanket which is not acceptable in most printing applications.
Another alternative is to use a blanket such as that described in U.S. Pat. No. 3,147,698 which incorporates a latex impregnated, heat set paper product as a compressible layer. This layer also serves as a cross machine direction stabilizing member due to its physical properties (low elongation and high modulus). This product has limited compressible properties making it undesirable in those cases where high resilience and high compressibility are required or desired.
The present invention solves the problem of cross machine elongation without significantly increasing the overall thickness of either the blanket or lower carcass layer, reducing the resiliency of the blanket or increasing the machine direction elongation characteristics of the blanket.